Bar screens can be installed at sewer pumping stations and the headworks of wastewater treatment plants. Rake screens are intended for the extraction of large and medium waste solids from industrial and municipal wastewater with subsequent unloading on a transporter or into a waste bin. The screens are intended for use in wastewater with pH = 6.5÷8.5.
Description of the Bar Screen
Rake screens of different dimensions are made of stainless steel AISI 304 or AISI 316 on client demand. Dimensions of the rake screens are selected depending on the required flow rate, wastewater composition and dimensions of the channels.
The rake screen is a filtering mesh consisting of bars installed into a frame. The bars are made from structural shapes with a teardrop-shaped cross-section that improves flow characteristics of the filtering mesh. The screen’s filtering mesh can also be made of rectangular or round profile bars.
Bar spacing in the filtering mesh can vary from 5 – 100mm for typical equipment or can be adjusted individually. The rectangular frame consists of two longitudinal boards connected by cross beams. The laminated stainless steel chains can be moved along polymer guides installed on the longitudinal boards of the screen. Chain tension is adjustable.
Cross rakes are installed on the chains for cleaning of the filtering mesh. Chains and rakes are actuated by shaft rotation with drive sprockets located in the screen top section. The shaft is driven by a geared motor. At the screen bottom, the chain is moved along a pilot slider made of a wear-resistant polymer material. Therefore, there are no rotating parts in the submerged section of the screen.
The screen is installed onto an axis of a special rotating prop mounted on the wall of the channel or onto a special platform fastened in the channel while the screen itself can be lifted over the channel for maintenance. If there is a lack of space in a short channel without the possibility of screen lifting, a special telescopic platform is used.
Operating principle of the Bar Screens
The rake screen collects and removes solids from wastewater that are larger than bar spacing. The solids are periodically taken off from the filtering mesh by the rake moving them to the top edge of the frame. The solids are then removed from the rake by means of a kicker and dispatched along the gravity slide onto a transporter or into a waste bin. The screen operation periodicity depends on the chosen equipment automation scheme and is defined by technological commission results.
Advantages of the Bar Screen
- Resistance to mechanical and hydraulic overloads due to high tensile design in comparison with other types of mechanized screens;
- High hydraulic efficiency is achieved by using tear-shaped bars that cause a pressure gradient to the mesh and across the bar spacing;
- Tear shaped profile as well protects the screen from waste jamming inside the filtering mesh. By using this design, such blockages can be removed by rakes allowing the screen to operate freely;
- High reliability achieved by the absence of the rotating parts in the submersible part;
- Submersible parts like casing, bars, chains and rakes are made of stainless steel to improve corrosion resistance in aggressive environments of wastewater;
- Improved ergonomics: maintainability of the screen due to accessibility of the main units in operation and screen lifting without demounting.
Automation of the Bar Screens
Screen automation can be done according to different parameters:
- Time period
- Wastewater level difference upstream and downstream of the screen
- Any customized program
Alarm systems are provided for normal operation indication, emergency stops, the wastewater top level in the channel and others. It is possible to transfer equipment status data to a higher-level automated process control system depending on the client’s requirements. The screen is supplied with a safety device jet lever to avoid damage to the screen such as chain stretching or breaking. The safety device and stops the screen operation in the event of any jam or mechanical hindrance to the chain movement. Light and sound alarms turn on simultaneously when the safety device actuates. The screens together with a conveyor, compacting press and slide gates are included in a high-performance complex for mechanical wastewater treatment. Equipping the control system with programmable modules makes it possible to integrate complex sub-systems into one system with a common control center and remote dispatching.